Abstract Project 1 will continue our investigation of masked hypertension (MHT), a potentially major public health problem. Defined by a normal clinic BP (CBP) and elevated ambulatory BP (ABP), MHT probably affects 10- 15% of the general adult population and is hypothesized to have an adverse prognosis. MHT is conceived as an intermediate condition between sustained normotension (SNT; normal CBP and ABP) and sustained hypertension (SHT; both elevated). Project 1's goal is to perform 5-year follow-up exams on a cross-sectional sample of 1000 healthy, initially employed subjects recruited from 4 worksites (treated hypertensives were excluded) whose baseline exam included CBP measured on 3 occasions, 24-hour ABP, demographic and psychosocial questionnaires, blood and urine measures of traditional and novel cardiovascular (CV) risk factors and mediators of the stress response, and assessment of vascular and cardiac structure and function. All tests will be repeated. This would address a major gap in knowledge: Does MHT lead to development of essential HT, progression of CV target organ damage (TOD), and increased risk of CV disease (CVD)? The Primary Aims are to test the following hypotheses: H1: The MHT effect {difference between mean awake ABP and CBP) at baseline prospectively predicts (a) risk of essential HT and (b) increases in CBP. H2: Persons with MHT at baseline are at increased risk for essential HT at follow-up, compared to those with similar CBP and normal ABP at baseline. H3: Those exhibiting a larger MHT effect at baseline will prospectively exhibit greater increases in markers of TOD and CVD risk burden than those with a smaller MHT effect. H4: Those with MHT at baseline will prospectively exhibit greater increases in markers of TOD and CVD risk burden than those with SNT.